The invention relates to a halogen incandescent lamp, comprising a light-transmitting lamp envelope which is sealed in a vacuum-tight manner and through whose wall current-supply conductors extend into the cavity of the lamp envelope where they are connected to the ends of a filament. The cavity is filled with a halogen-containing inert gas. Such a lamp is known from, for example, British Patent Specification 2,025,127.
It is advantageous to design halogen incandescent lamps for use at a low voltage because the filament then has small dimensions and the emitted light can then be concentrated very effectively. The term "low voltage" is to be understood herein to mean a voltage which is at most half the mains voltage. If, however, a large number of low-voltage lamps has to be used, this has the disadvantage that many transformers are required or that, when only one or a few transformers are employed, very high currents flow in the secondary circuit.
These disadvantages could be avoided if the lamps were operated in series-combination at mains voltage, but the risk of lamp explosions then occurs. In fact, if the filament of a lamp of the series burns through at the end of its life, a discharge arc may be obtained. In the long run, this arc may swell, touch the wall of the lamp envelope and may overheat that wall, after which the lamp envelope explodes.
The risk of explosion is not eliminated by including a fuse in the current circuit because the remaining lamps in the series-combination limit the current through the circuit so that this current is not or substantially not larger when a discharge arc has been obtained in a lamp.
Even if the gas filling of the lamp is chosen so that the discharge arc has a high re-ignition voltage, the re-ignition of the arc cannot be prevented. This is because, after each zero passage of the voltage, mains voltage is applied across the defective lamp before a current starts to flow again through the circuit. Moreover, lamps operated at a low voltage are generally small and the distance between the points to which a discharge arc applies is consequently also small.